Electronic or electrical devices usually operate with suitable energy sources connected thereto. Common energy sources may include electrical sockets in a power grid, energy harvesting devices, fuel cells and batteries.
Energy harvesting devices such as photovoltaic cell or piezoelectric energy harvester may be used conveniently in daily life to convert energy of certain types into electrical energy for powering up electrical devices. Energy harvesting devices may be designed to operate with one or more optimal conditions such that the energy harvesting efficiency may be maximized during operations.